Warming and humidification of inspired gases : its effectiveness in minimizing hypothermia in anaesthetized cats : a thesis presented in partial fulfilment of the requirements for the degree of Master of Veterinary Science at Massey University

Copyright is owned by the Author of the thesis. Permission is given for

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the permission of the Author.

ITS EFFECTIVENESS IN MINIMIZING HYPOTIIEinflA IN ANAESTHETIZED CATS

A THESIS

PRESENTED IN PARTIAL FULFILMENT OF THE REQillREMENTS FOR THE DEGREE OF

MASTER OF VETERINARY SCIENCE AT

MASSEY UNIVERSITY

ROSLYN MACRON 1993

11

ABSTRACT

Body temperature is governed by a complex, highly integrated control system which carefully balances heat production and heat loss. Heat is produced as a byproduct of metabolism, and as the result of muscular work, shivering and chemical thermogenesis;

while heat is lost from the body via the channels of heat exchange - radiation, conduction, convection and evaporation. General anaesthetic agents interfere with the normal mechanisms of temperature control by reducing heat production in the face of increased heat loss.

Six adult domestic short-haired cats were included in a randomized cross-over study, to evaluate the effectiveness of warming and humidification of inspired gases in the prevention of anaesthetic induced hypothermia. General anaesthesia was maintained with halothane in

100%

oxygen, delivered via a Mapelson type E non-rebreathing anaesthetic circuit. Both passive and active methods of inspired gas warming and humidification were investigated in this study: the passive technique evaluated the effectiveness of a human neonatal Heat and Moisture Exchanger (HME), while the active technique used an electrical heating unit to supplement the warming capabilities of the HME.

Rectal and oesophageal temperatures continued to fall throughout each of the

120

minute experimental periods. Body temperature did not vary significantly between the three trials. The effectiveness of the HME in preserving normothermia in anaesthetized animals has not been reported previously. Despite the success of similar techniques in human neonates and infants, the results of this study indicate that warming and humidification of inspired gases is ineffective in minimizing hypothermia in halothane anaesthetized cats.

PREFACE AND ACKNOWLEDGEMENTS

Heartfelt thanks are extended to the following for their help and support:

My supervisors, Prof. Elwyn Firth and especially, Assoc. Prof. Brian Goulden, who boldly stepped in when everyone else stepped out;

Anne Field-Mitchell and Sarah Fort for their time and effort spent preparing various stages of the manuscript, and for helping me meet "rush job" deadlines;

Helen Hodge (nee Stephens) for willingly giving up her weekends;

Vickie King (CVM, University of Minnesota, St Paul MN USA), statistical goddess;

and my husband Brian Anderson; cat-holderer extraordinaire, weekend worker and computer whiz-kid, who cajoled, bullied and occasionally threatened me in order to help complete this project.

iv

TABLE OF CONTENTS

Preface and Acknowledgements

^.

. . . iii

Figures and Tables . . . vii

Introduction

^.

. . . 1

Literature Review

1

Temperature Regulation

^.

. . . 3

1: 1 The Thermal Steady State . . . 3

1:2

Endothermy

^{. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .}

5

1:3 Heat Production . . . 6

1 :3(i) "Muscle" Heat . . . 6

1: 3(ii) Shivering . . . 7

1:3(iii) Non-shivering Thermogenesis & Enhanced Cellular Metabolism . . . 7

1:4 Heat Loss . . . 9

1:5 1 :4(i) Conduction . . . 9

1 :4(ii) Convection . . . .. . . ... 10

1 :4(iii) Radiation

^{. . . . . .}

.

^{. .}

.

^{. .}

.

^{. . . . . . . . . . . . .}

10

1 :4(iv) Evaporation ... 1 1 Body Temperature 1 :5(i) 1 :5(ii)

. . . .

.

^{. .}

.

^.

. .

^{. . . . . . . . . . . . . . .}

.

^{. .}

.. . . . 1

2

The Core and the Shell . . . 13

Representative Body Temperatures ... 14

1:5(ii)(a) Oral Temperature ... 16

1:5(ii)(b) Rectal Temperature ... 16

1:5(ii)(c) Oesophageal Temperature ... 17

1:5(ii)(d) Tympanic Membrane 1:5(ii)(e) 1:5(ii)(f) Temperature . . . 18

Bladder Temperature ... 19

Pulmonary Artery Temperature ... 19

1:6 Control of Body Temperature ... 20

1:6(i) Thermoregulatory Sensors ...

^.

... 20

1:6(ii) Processing & Integration of Thermal Information

^{. .}

22

1:6(iii) Thermoregulatory Effectors ...

^.

.. 24

2 The Effects of Anaesthesia on Temperature Regulation . . . 27

2: 1 Mechanisms of Perioperative Heat Loss . . . . . . . . 27

2: l(i) Radient Heat Loss . . . 27

2: l (ii) Conductive Heat Loss . . . 28

2: l (iii) Convective Heat Loss . . . 29

2: 1 (iv) Evaporative Heat Loss . . . . . . 29

2:2 Thermoregulation During Anaesthesia . . . 30

2:3 The Effects of Anaesthetic Agents on Temperature Regulation . . . 3 1

3

Hypothermia

&

Its Effects on the Body . . . 34

3: 1 The Effect of Hypothermia on Metabolism . . . 34

3:2 The Effect of Hypothermia on the Cardiovascular System . . . 35

3:2(i) The Heart . . . 35

3:2(ii) The Circulatory System . . . 36

3:3 The Effects of Hypothermia on the Respiratory System . . . 37

3:4 The Effects of Hypothermia on the Central Nervous System . . . 37

3:5 The Effects of Hypothermia on Neuromuscular Function . . . 38

3: 6 The Effects of Hypothermia on Hepatic & Renal Function . . . 39

3:7 The Effects of Hypothermia on Fluid, Electrolyte & Acid-Base Status . . . .. . . 39

3: 8 The Effects of Hypothermia on Coagulation . . . 40

3:9 The Effects of Hypothermia on the Immune System . . . 4 1 3: 10 The Effects of Intraoperative Hypothermia on the Anaesthetized Patient . . . . . . 41

3: 1 O(i) The Intraoperative Period . . . . . . . . . . . . . . . . 42

3: lO (ii) The Postoperative Period: The Metabolic Cost of "Shivering" . .... . . .... . . . ... . .. 43

4 Prevention of lntraoperative Hypothermia . . . . .. . . 47

4: 1 Controlling Radiant Heat-Loss . . ... . . .. . . . .. . . .... . 48

4: l (i) Control of Environmental Temperature . . . . .... 48

4:2 Controlling Conductive Heat-Loss . . . .... . . ... 48

4:2(i) Heating Blankets (Mattresses) ... . . . . ... 49

4:2(ii) Fluid Warmers . . .... . ... . . . .. 49

4:2(iii) Additional Methods of Controlling

Conductive Heat-Loss . ... . ...

50

vi

4:3 Controlling Convective Heat-Loss

^{. . . . . . . . . . .}

. . .

^{. . . . . . . . . . .}

50

4:4 Controlling Evaporative Heat-Loss

^{. . . . . . . . . . . . . . . .}

.

^.

. . .

^{. . .}

5 1 4:4(i) Airway Warming & Humidification

^{. . . . .}

.

^{. .}

.

^.

5 1 4:4(ii) Active Warming & Humidification of Inspired Gases

^.

.

^{. .}

.

^{. .}

.

^{. . . . .}

.

^{. .}

. .

^{. . .}

52

4:4(iii) Passive Warming & Humidification of Inspired Gases

^.

. . . 54

Summary

^{. . . . . . . . . . . . .}

.

^{. . . . . . . . . .}

. .

^{. . . .}

.

^.

.

^{. . . . . . . . .}

.

^.

56

Materials and Methods

^.

. . . . 57

Experimental Design

^.

. . . . 57

Collection of Data

^.

. . . . 66

Analysis of Data

^{. . . . . . . .}

.

^{. . . . . . . . . . . . . . . . . . . . . . . . . .}

67

Results

^{. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .}

69

Disscussion

^{. . . . . . . . . . . . . . . .}

. .

^{. . . . . . . . . . . . . . . . . . . . . . . . .}

74

Appendix

^.

. . . . 80

Bibliography

^.

. . . . 99

FIGURES AND TABLES

Figure 1. The "Thermal Balance" . . . . . . . 4

Figure 2. Thermoregulatory Control: the interthreshold range and the thresholds for warm and cold responses . . . 23

Figure 3a. Humid Vent Mini, Heat and Moisture Exchanger .

. . . 60

Figure 3b. HME cutaway displaying the hygroscopic paper microwell responsible for warming and humidification

of inspired gases

.

. . . 60

Figure 4. Humid Vent Mini,

HME

attached to the endotracheal tube adaptor . . . . . . . 61

Figure 5 . The experimental breathing circuit; a human neonatal Mapelson type E system attached to the

electrical heating unit . . . . . . . 63

Figure 6. Mercury-in-glass thermometer positioned at the fresh gas outlet of the experimental circuit . . . . 64

Figure 7. Mercury-in-glass thermometer positioned at the

Y

-piece of the experimental circuit . . . . 64

Figure 8. Mean (

+

SEM) Rectal and Oesophageal Temperatures Trial

I

. . . 71

Figure 9.

Mean ( ± SEM) Rectal and Oesophageal Temperatures Trial

II

72 Figure 10. Mean ( ± SEM) Rectal and Oesophageal Temperatures Trial

m

73 Figure 11. Trial

I

Figure 12. Trial

II

Figure 13.

Trial m

Figure 14. Trial

I Figure 15. Trial II Figure 16. Trial m

Oesophageal Temperature (degrees celsius) . . .

^.

. . 82

Oesophageal Temperature (degrees celsius) ... 84

Oesophageal Temperature (degrees celsius) . . . . . .

86

Rectal Temperature (degrees celsius) . . . 88

Rectal Temperature (degrees celsius) . . . . . .

^.

. .

90

Rectal Temperature (degrees celsius) . . . 92

vw

Table

I.

Table

II.

Table

Ill.

Mean Rectal

and

Oesophageal Temperatures for Trial

I

. . . 71

Mean Rectal

and

Oesophageal Temperatures for Trial

II

. . . 72

Mean Rectal

and

Oesophageal Temperatures for Trial

Ill

. . . 73

Table

IV.

Trial

I

Table

V.

Trial

II

Table

VI.

Trial

ill

Table

VII.

Trial I Table

VIII. Trial II

Table

IX .

Trial

III

Table

X.

Trial

I

Table

XI.

Trial

I

Table

XII.

Trial

II

Table

XIII .

Trial

II

Table

XIV. Trial III

Table

XV.

Trial

m

Oesophageal Temperature °C . . . 81

Oesophageal Temperature °C

.

. . . 83

Oesophageal Temperature °C

.

. . . 85

Rectal Temperature °C . . . . . . 87

Rectal Temperature °C

.

. . . 89

Rectal Temperature °C . . . 91

Respiratory Rate (breaths per minute)

...^.

.

.^. . .

93

Heart Rate (beats per minute)

..

..

.

.

...

.

. .

94

Respiratory Rate (breaths per minute) ..

.

...

.

.

.

95

Heart Rate (beats per minute)

..

..

.. . ...

..

. .

96

Respiratory Rate (breaths per minute)

.^. .

.

. . .

.. 97

Heart Rate (beats per minute) .

. . . .... . ...

.. 98